In magnetic memory disks, information is recorded in concentric tracks on both sides of a disk. Read/write heads are moved to a particular track to gain access to the information recorded in that track. The tracks on a side of a magnetic disk are extremely closely spaced, making it difficult to meet the requirement for high-speed movement of a read/write head. Requirement of head movement within a 15 microsecond time window is typical.
Until recently, most read positioning of read/write heads has been provided by a linear actuator which moves the heads radially along the disk. Thus, in most instances the heads and the carriage which supports the heads move across the disk in a straight line manner. As noted in U.S. Pat. No. 3,984,873 to Pejcha, one disadvantage of the linear actuator involves the inertia which must be overcome during rapid acceleration or deceleration of the carriage which supports the heads. Ringing, i.e. oscillation of the heads following deceleration, is detrimental to efforts to minimize the time required for head repositioning.
The Pejcha patent teaches an actuator which moves the head along an arcuate path across a disk. The rotary actuator mounts the heads at the end of an arm assembly which is pivoted at the opposite end. The heads therefore are moved about the maximum radius of the apparatus so that higher acceleration can be gained for quick access to the desired information. Moreover, the end of the arm supporting the heads presents less inertial resistance to movement, thereby permitting rapid acceleration between positions.
While the industry has been moving toward use of rotary actuators over linear actuators, quality control in the production of magnetic memory disks is still performed using linear actuators. One step in the testing of memory disks is the recordation of data on the disk to verify that the disk will indeed accept information. Clearly, it is best to perform such verification by use of an actuator which simulates the characteristics of an actuator of eventual use. Because rotary actuators include arcuate translation to move heads, there is an angular error to consider which need not be considered with respect to linear actuators.
Because accuracy is critical, the prior art teaches that the arm supporting a read/write head is to be driven directly by the motor, thereby eliminating ball bearing assemblies which would be necessary if the motor were to drive rollers. Bearing assemblies require spacing tolerances that lead to errors. Direct drive of the arm, however, limits the torque which may be applied to the arm by the motor since maximum torque is increased as the motor is distanced from the arm to be pivoted.
It is an object of the present invention to provide a rotary actuator to be used in memory disk testing which incorporates incremental stepping functions to simulate disk drive characteristics with extreme accuracy and repeatability.